1. Field of the Invention
The present invention relates to an ink jet printing apparatus that forms an image on a print medium by use of a print head to eject ink from a plurality of printing elements arranged with density. More particularly, the invention relates to a method of controlling a print head of a serial-type ink jet printing apparatus that ejects ink while scanning the print head relative to the print medium.
2. Description of the Related Art
In the serial-type ink jet printing apparatus, an image is to be formed by alternately performing main scan for the carriage mounting a print head to make a printing while scanning parallel with a surface of a print medium and conveyance operation to feed the print medium in a direction transverse to the main scan. On the print head applicable for such a printing apparatus, a multiplicity of printing elements are arranged at a predetermined arrangement density in a direction transverse to the main scan in order to eject ink depending upon print information.
Japanese Patent Laid-Open No. S54-51837 discloses an ink jet print head of a scheme to eject ink by utilization of thermal energy. According to the print head in the document, each of its printing elements is structured with ejection ports through which ink is to be ejected, an ink path for guiding ink to a vicinity of the ejection ports, and an electrothermal conversion element (heater) arranged in the ink path. By applying a voltage pulse to the electrothermal conversion elements depending upon image data, film boiling is caused in the ink contacting therewith. By the growth action of bubbles produced, droplets are ejected through the ejection ports.
Meanwhile, Japanese Patent Laid-Open No. H5-330066 discloses a novel structure of a print head that is further increased in the arrangement density of the printing elements and capable of ejecting ink droplets in a slight amount at high frequency with the utilization of thermal energy similarly to Japanese Patent Laid-Open No. S54-51837, in order to meet the requirement to output a precise image at high speed. Recently, image output has been available with high definition at high speed but less granularity by adopting the structure as disclosed in Japanese Patent Laid-Open No. H5-330066.
However, it is confirmed that an air flow occurs between the print head and the print medium and has an effect upon the direction of ejecting ink droplets, on the print head arranged densely with individual print elements and capable of ejecting small droplets of ink at high frequency. Specifically, out of a plurality of printing element arrays arranged in a predetermined direction, there encounters a phenomenon that the ink, ejected from the printing element located close to an end thereof, is deflected toward a printing element located centrally.
FIG. 1 is a figure for typically explaining the adverse effect upon an image. This illustrates a print state on a print medium where a uniform image is printed by performing print scan once. The ink droplet, ejected from an ejection port located at the end of the print head, deflects in a manner attracted toward the center and arrives at the print medium, with a result that tone value is higher centrally than that at the end region. The image area thus formed, if continued in the sub-scan direction, raises a band-like tone unevenness over the entire image. From now on, such phenomenon is referred to as end-deviation phenomenon, for the sake of convenience.
The degree of such end-deviation phenomenon increases with the increase of the arrangement density of printing elements on the print head, with the increase of drive frequency and with the decrease of ejection volume (droplet volume). Meanwhile, it is also under the influence of the carriage moving speed and the distance between a print medium and an ejection-port formed surface (hereinafter, referred to as head-medium distance).
However, such ink deflection as to cause an end deviation can be suppressed to a certain extent by adopting a multi-pass printing method. The multi-pass printing method refers to a method that the print data, which can be printed by performing one print scan of the print head, is divided into a plurality of print scans, thereby completing an image phase by phase. The adoption of the multi-pass printing method reduces the print data for performing one main print scan, thus making it possible to reduce the substantial drive frequency to the print head and to suppress the occurrence of end deviations. As the number of multi-pass, i.e., the number of divisions of data which can be printed by performing one main print scan, increases, the reduction effect of end-deviation phenomenon can be obtained to a greater extent.
Japanese Patent Laid-Open No. 2002-096455 discloses a printing method to make such an end-deviation phenomenon inconspicuous with further actions. The multi-pass printing method usually uses a mask pattern defining the permission/non-permission to print in pixel in order to define the position of the data permitted to print by performing one main print scan. Japanese Patent Laid-Open No. 2002-096455 discloses a mask pattern in which the print permission ratio, corresponding to the printing element located closer to the end, is suppressed lower than the print ratio corresponding to the printing element located centrally. The use of such a mask pattern makes it possible to output an image excellent in uniformity through the effect to actively suppress the ejection frequency at a printing element ready to cause ink droplet deviation, in conjunction with the effect of the usual multi-pass printing method.
However, in the multi-pass printing method, the area which can be printed by performing print main scan once is completed by a plurality of cycles of print scans, thus increasing the time required in printing and incurring the lowering of throughput.